Fig. 1974.
In [Fig. 1974], for example, we have at r a cylinder with a right-hand groove in it, whose side c, representing the face of a tooth, is supposed to be a radial line from the cylinder axis, the side b representing the back of a cutter tooth, being at an angle of 40°.
Now if the work revolves in the direction of arrow a, and the cut be started at end g (as it must to cut a right-hand groove with the work revolving as at a), then the side c of the groove will move over towards and upon the side of the cutter for the reasons explained with reference to [Fig. 1969], and the teeth on this side being at the least angle to the side of the cutter, do not clear the cut so well, the teeth doing some cutting after passing their lowest point of revolution—or in other words, after passing the line g in [Fig. 1968]. The effect of this is to cause the cutter to drag, as it is termed, producing a less smooth surface on that side (c) of the groove or tooth.
We may, however, for a right-hand groove revolve cylinder r, as denoted by arrow e, and start the cut at end d. The result of this is that the side c of the groove, as the roller revolves, moves away from the side of the cutter, whose teeth therefore do no cutting after passing their lowest point of revolution (g, [Fig. 1968]), and the dragging action is therefore avoided, and the cut smoother on this which is the most important side of the tooth, since it is the one possessing the cutting edge. When “dragging” takes place the burr that was shown in [Fig. 1965] at d, is formed, and must, as stated with reference to that figure, be removed either by filing or grinding.
Obviously if the direction of cutter revolution and of feed is arranged to cause side c to move away from the side of the cutter, then side b will move over towards the other side of the cutter; but on account of the cutter teeth on this side being at a greater angle to the side of the cutter, they clear better, as was explained with reference to [Fig. 1968], and the dragging effect caused by the revolving of the work is therefore reduced.
Fig. 1975.